At the present time, conventional arc lamps are providing the light source for light fastness testers are constructed as shown in FIG. 1. The lower electrode holding part 1 has four carbon electrodes mounted thereon, and an upper electrode holding part likewise has four carbon electrodes mounted thereon directed downward toward the carbon electrodes mounted on the lower holding part 1. Light discharge between the upper and lower electrodes occurs when the distance between the two holding parts is automatically adjusted by means of a servo motor 5, a chain 3 and a gear 4. A specimen to be tested for light fastness is placed in the vicinity of the lamp.
Conventional carbon electrodes used in the above described arc lamp generally have a light emitting material or a central core which is surrounded by carbon material. The carbon material is, itself, coated with a copper coating around the sides and across the bottom. The tip of the conventional electrode is not copper-coated.
The problem with these conventional carbon electrodes is that they have a relatively short burning life, and more importantly, they do not have suitably stable light emitting qualities. As the carbon tube material of these conventional electrodes is consumed during discharge between the electrodes, the discharge emitting the light moves from the outer carbon toward the central core, thus changing the location of the bright spot of the emitted light. As the bright spot moves, the properties of the emitted light differs spectrally.